by Bob Tuzik, January 1, 2005
When NYCTA wanted a new fastener that was 35% “softer” than the softest DF fastener it had in use, a fastener that would not raise the track by more than 1-1/8 inches (30 mm), Dynamic Engineering designed the APT-BF system. The APT-BF fastener incorporates two elastic layers: a rail pad and a base plate pad. The design utilizes a very resilient base plate pad that is “pre-loaded” to 90% of the maximum vertical fastener load through the use of two compression springs that are placed over the anchor bolts that extend through the rail base plate. The springs are typically compressed by 3 to 5 mm from their free length to generate the pre-load that in turn compresses the very resilient base plate pad. Passing wheel loads compress the base plate pad further, eliminating the pre-load on the base plate pad and producing a limited (generally below 3 mm) vertical rail displacement. This provides a vertical dynamic stiffness low enough for good vibration mitigation, but with a vertical static stiffness high enough to assure good track stability.
Since the fastener had to be delivered as a single unit, it is assembled on a steel plate that is bolted to the wooden sleepers. The steel plate is 1/2-inch (12-mm) thick to provide a secure anchor for the 1-inch (25-mm) diameter studs that form the backbone of the pre-load system. The maximum 3/4-inch (19-mm) thick vibration damping material consists of a 3-mm-thick rubber sheet studded with natural rubber discs with a diameter of 35 mm and a height of 19 mm.
The APT-BF fasteners installed in the Grand Street subway station in Manhattan and on the elevated F-line in Brooklyn have provided an average 20 dB reduction in vibration levels at frequencies between 25 Hz and 120 Hz, compared to the standard direct fixation track. Variations of the design have demonstrated to be effective solutions to combat wheel squeal, ground-borne noise and corrugation.